Isolation of a Reactive Cyclopropane Intermediate via a Unique Catalyst‐Free Spontaneous Cyclopropanation Step at 0 °C

Abstract

A remarkably mild, catalyst-free cyclopropanation reaction is confirmed by low-temperature isolation of a fused bicyclic cyclopropane intermediate. This intermediate is one of two that were isolated and fully characterised in a three-step temperature-controllable reaction cascade.

Tricoordinate Beryllium Radicals and Their Reactivity

“…It likely results from the overreduction of 2-Et 2 O , followed by C–H activation by the anionic Be­(I) center of one C2-methyl substituent at the carbene center to form the fused cyclopropane ring, and finally addition of the anionic Be–H moiety to the C carbene –N bond of the CAAC ligand, leading to the six-membered C 4 BeN ring. This kind of CAAC ring expansion has been observed with CAAC-stabilized boranes and low-valent boron compounds, − while the spontaneous cyclopropanation by C2-methyl C–H activation has been reported upon adduct formation of CAAC with 2-mesityl-1- tert -butyliminoborane . In contrast, the twofold reduction of [(CAAC)­MgCl 2 ] in the presence of another equivalent of CAAC resulted in the rearrangement of one CAAC ligand to a cyclobutanimine, into which the Mg center inserts to give a C 4 Mg metallacycle with a pendant imine functionality .…”

“…This kind of CAAC ring expansion has been observed with CAAC-stabilized boranes and low-valent boron compounds, 41−43 while the spontaneous cyclopropanation by C2methyl C−H activation has been reported upon adduct formation of CAAC with 2-mesityl-1-tert-butyliminoborane. 44 In contrast, the twofold reduction of [(CAAC)MgCl 2 ] in the presence of another equivalent of CAAC resulted in the rearrangement of one CAAC ligand to a cyclobutanimine, into which the Mg center inserts to give a C 4 Mg metallacycle with a pendant imine functionality. 45 The 9 Be NMR shift of 3 at 17.4 ppm (fwmh ≈ 257 Hz) is consistent with a trialkylberyllate.…”

1,3-C–H bond activation on a transient gallium(i)/isocyanate adduct

“…Braunschweig et al reported an unusual C–H addition of an unsaturated NHC's backbone to a BN bond of an iminoborane, affording a borylated carbene. 6 The Braunschweig group also reported C–H addition of acetone across BN triple bond and across BB of an NHC-ligated diboryne. 7 Schulz and co-workers also reported the enolization of ketones by a gallium phosphinidene.…”

Synthesis of Iminoboryl o-Carboranes by Lewis Base Promoted Aminoborirane-to-Iminoborane Isomerization

“…Since then, BN-containing compounds have attracted tremendous attention, as the replacement of CC with BN bonds has turned out to be an ingenious strategy to create novel molecules featuring unique chemical and physical properties. A variety of iminoboranes have been synthesized, allowing the expansion of the reactivity patterns − over the past half century, including adduct formation, cycloaddition with MC, EC (E = O, S, Se), CC, and RN 3 , and polar bond addition. ,,, Very recently, the Kong group reported boraiminolithium, which displayed potential in the synthesis of N-functionalized iminoboranes . However, the variation of substituents on the boron site is still of great challenge, and the scope of B-substituents is limited to alkyl, aryl, amino, silyl groups, and transition-metal fragments (Figure a).…”